The human spinal column includes a number of vertebrae of different sizes. The cervical vertebrae, forming the neck area, are relatively small. Just below the cervical vertebrae are the thoracic vertebrae, which form the upper back. The thoracic vertebrae are larger than the cervical vertebrae, and increase in size from top to bottom. Below the thoracic region lie the lumbar vertebrae, which are still larger. Injuries to different parts of the spine may necessitate different types of treatment, based on the type and extent of the injury as well as the size and density of bone at the site of the injury. For instance, certain spinal injuries or deformities require fixation or immobilization of adjacent vertebrae, with rigid members of appropriate stiffness anchored to the vertebrae in order to brace them and limit movement.
Various devices for internal fixation of bone segments in the human or animal body are known in the art. For instance, pedicle screw and/or hook systems are sometimes used as an adjunct to spinal fusion surgery, and provide a means of securing spinal rods or other elongate members to two or more vertebrae. Such systems may have a rod-receiving portion and an integral anchor portion, or may be provided with a separate anchor member, especially one that may be pivoted with respect to a rod-receiving member. The rod-receiving portions of the devices (also referred to as coupling devices) couple to the pedicle screw or hook and receive an elongate members such as a spinal rod (commonly referred to as a distraction rod). Two or more rod receiving devices are inserted into respective vertebrae and adjusted along the spinal rod to distract, de-rotate, and/or stabilize a spinal column, for instance to correct scoliosis or stabilize the spinal column in conjunction with an operation to correct a herniated disk. One goal of such a system is to substantially reduce and/or prevent relative motion between the spinal segments that are being fused.
The size, positioning, and curvature of the cervical spine present surgeons with different challenges than the lumbar spine. For instance, since the cervical vertebrae are relatively small and spaced closely together, the devices used to anchor a spinal rod to the bone must be small enough to be placed in close proximity without abutting one another. Furthermore, anchoring a spinal rod to cervical vertebrae with large screws or other anchor devices designed for lumbar use may destroy or irreparably damage the small cervical vertebrae. Therefore, smaller anchor members are usually utilized in the cervical region. In addition, the gauge or stiffness of the spinal rods used in the cervical region ordinarily differs from that used in the thoracic or lumbar regions, since a larger and less flexible spinal rod may provide enough force to pull anchor members out of cervical vertebral bone.
In many cases where an immobilization system must span the cervical and thoracic vertebrae, and potentially the lumbar vertebrae as well, the ability to connect a smaller diameter cervical spinal rod with a larger diameter thoracic/lumbar spinal rod must be provided. Specialized spinal rods that transition from a narrower portion for cervical use to a wider portion for thoracic/lumbar use have been designed. However, those rods usually have a long tapered portion to transition from one region to another. Since this tapered transition portion has a constantly changing diameter, coupling devices may not be secured to the rod along the transition portion, creating a large amount of unusable space along the length of the spinal rod. Since spinal rods are usually made of relatively inflexible materials, such as titanium or stainless steel, without a long transition portion the abrupt transition between the narrower, more flexible cervical portion and the wider, stiffer thoracic/lumbar portion will create significant stress at the transition juncture, which could cause sheering of the rod at that point.